On a commercial scale, hydrogen chloride is passed into 20 per cent oleum, until saturation is reached. This is heated in a nitric acid still, when the chlorosulfonic acid distills over between 150°-160° C. With 30 per cent oleum, the conversion factor is about 42 per cent. The residue in the still is about 98 per cent sulfuric acid. It forms a colorless liquid, boiling at 152° C., and having a density of 1.7. Chlorosulfonic acid fumes in the air, because reaction with water forms sulfuric acid and hydrochloric acid. ClSO₂OH + H₂O = H₂SO₄ + HCl This material was not used by the United States since oleum was found superior. =Oleum.= Oleum is a solution of 20 to 30 per cent sulfur trioxide (SO₃) in concentrated sulfuric acid. It has been used by the Germans to produce clouds on land and sea, by its contact with quicklime, and by the Americans for screening tanks and aeroplanes. Sulfur trioxide has been found to be superior as a shell filling. It is believed that the smoke producing power of oleum is due solely to its sulfur trioxide content, the sulfuric acid itself acting only as a solvent. The rather high freezing point of the oleum containing high percentages of sulfur trioxide is a disadvantage. =Sulfur Trioxide.= Sulfur trioxide, SO₃, is a colorless mobile liquid, which boils at 46° C. and solidifies to a transparent ice-like mass, melting at 15° C. It is prepared by passing a mixture of sulfur dioxide and oxygen over finely divided platinum or other catalysts at a temperature between 400 and 450° C. Sulfur trioxide can only be used as a filler for shell and bombs, and is probably the best substitute for phosphorus. =Tin Tetrachloride.= Tin tetrachloride, SnCl₄, is obtained by the action of chlorine on metallic tin. It is a liquid, boiling at 114° C., and having a density of 2.2. It fumes in the air, because it hydrolyzes to stannic hydroxide: SnCl₄ + H₂O = Sn(OH)₄ + 4 HCl It makes a better and more irritating smoke for shell and hand grenades, than either silicon or titanium tetrachlorides. Since there is practically no tin in this country, the other tetrachlorides were developed as substitutes. =Silicon Tetrachloride.= Silicon tetrachloride, SiCl₄, is prepared from silicon or from impure silicon carbide by heating it with chlorine in an electric furnace. The raw material (silicon carbide) is a by-product in the manufacture of carborundum. It is a colorless liquid, boiling at about 58° C., and fumes in moist air, owing to hydrolysis: SiCl₄ + 4 H₂O = Si(OH)₄ + 4 HCl It is not very valuable in shell, though it is more effective on moist, cool days than on warm, dry ones. Its greatest use is found in the smoke cylinder, combined with ammonia. By the action of the moisture of the air, the following reaction takes place: SiCl₄ + 4 NH₃ + 4 H₂O = Si(OH)₄ + 4NH₄Cl The addition of a lachrymator gives a mixture which works well in hand grenades for mopping up trenches.